In recent years, high quality images have been increasingly demanded of image forming apparatuses such as copiers, printers, and the like which carry out digital exposure. Further, digital imaging has progressed in which electrostatic latent images are formed employing digital exposure and subsequently developed.
In common images, the ratio of an area, to which a toner is practically applied to carry out printing, is no more than 30 percent with respect to the entire image area. In digital exposure, being different from analogue exposure, it is easy to carry out exposure in which image information signals are reversed. Accordingly, from the viewpoint of print speed as well as minimization of the fatigue of a photoreceptor, it is advantageous that parts of an image are subjected to exposure and to reversal development. However, the reversal development is unstable as the development method, compared to the normal development, and as a result, it is difficult to carry out stable reversal development.
Prior to the development of a digital image, image exposure is carried out by controlling light intensity as well as the exposure time employing a semiconductor laser, LED, and the like, to form latent image dots. Accordingly, laser dots are composed of electrical potentials distributed in a normal distribution-like state. However, when the latent image dots formed in such a state are developed, it is required that the size and shape of dots in the original document are the same as those in the developing image. Specifically, when halftone, and the like, is printed, image quality is determined depending on the degree of matching the dots of the developing image to those of the original document.
When a common toner is employed, it is impossible to carry out stable development with high reproducibility for electric potential having a semi-normal distribution when such a distribution exists at development. The reasons for this are as follows. In commonly employed toner, which is prepared employing a pulverization method, fractures exist on its surface, and minute toner particles, which remain unclassified, exist, or minute toner particles, generated by stress in a development unit, remain. Accordingly, the distribution of electrostatic potential is widened, and toner having low electrostatic potential or toner components having reverse polarity, are adhered onto the edge portion of dots. Thus, it is impossible to form dots having uniform size as well as shape.